1. Field of the Invention
The present invention generally relates to high performance transistors suitable for manufacture at very high density in integrated circuits and, more particularly, to transistors formed with performance enhancing structures below the gates thereof.
2. Description of the Prior Art
The potential for improved performance, functionality and economy of manufacture has driven continual increases in integration density of integrated circuits and reduction of size of individual transistors therein. However, the electrical characteristics of transistors are difficult to maintain when fabricated at smaller feature sizes. For example, in field effect transistors (FETs), short channel effects and punch-through, differences in conduction between nFET and pFET devices and differences of impurity diffusion rates have led to sophisticated impurity structures and profiles to control the field in and adjacent to the channel and below the gate to manipulate the channel geometry and maintain acceptable on/off resistance ratios at low control voltages (e.g. below breakdown thresholds which are reduced at reduced dimensions).
Another technique of regulating channel dimensions, particularly channel depth, and junction capacitance (which, in bulk semiconductor devices is large and degrades switching speed) is through use of silicon-on-insulator (SOI) or ultra-thin silicon-on-insulator (UT-SOI) substrates on which the transistors are formed. UT-SOI technology can avoid the problem that, in bulk semiconductor devices at small sizes, the channel is too deep to allow adequate control of short channel effects. However, the thin silicon layer in SOI devices and UT-SOI device, in particular, causes increased resistance which is very difficult to reduce without a trade-off of increased susceptibility to short channel effects, particularly between nFETs and pFETs in complementary (e.g. CMOS) circuits due to differences in diffusivity of boron and arsenic or phosphorus and which generally require different set-backs or recess differences of source/drain and extension regions for optimal nFET and pFET designs that are not generally practical to provide. Also, the insulator layer in SOI substrates prevents effective electrical connection to the channel regions and results in floating body effects which can unpredictably alter the switching threshold of transistors. Further, UT-SOI wafers are far more expensive than bulk semiconductor wafers and significantly increase the cost of manufacture of integrated circuit chips.
A known technique for reducing resistance in thin SOI film transistors is to provide a raised source and drain (RSD) structure by growth of additional semiconductor material in the source and drain regions. However, RSD structures are generally formed adjacent a thin spacer on the sides of the transistor gate and increase the overlap capacitance (the capacitance between the extension impurity region and the gate electrode across the gate dielectric and thin spacer) significantly; degrading transistor performance. Typical capacitance increases for a 30 nm RSD are about 0.08 fF/μm (about 25% increase) for a 10 nm oxide spacer and about 0.2 fF/μm (about 50% increase) for a 10 nm nitride spacer. Additionally, the minimal thickness of the spacer appropriate to reducing resistance through the use of RSD structures places the source/drain implants too close to the gate.
It is also known that electrical properties may vary substantially between pFET and nFET devices due to differences in carrier mobility. It is also known that carrier mobility can be altered by application of tensile or compressive stresses to a volume of semiconductor material. However, the application of stresses to transistor designs without causing other undesirable effects such as warping of the chip is difficult and complicated even when the stress is applied from a stressed film formed over a conventional transistor design. It has only recently become practical to provide both tensile and compressive films at respective locations on the same chip. Further, forces applied from films formed over or even around transistor structures transfer forces to a semiconductor substrate or layer indirectly in shear (causing opposite stressing of adjacent regions) and the pattern of forces within a substrate or other semiconductor layer rapidly diminish with depth and are difficult to regulate while being more likely to cause chip warping. No technique is known for developing controlled tensile or compressive forces within a semiconductor substrate or layer to directly stress a desired region within that semiconductor substrate or layer.